Method for electroforming a copper article



United States Patent METHOD FOR ELECTROFORMING A COPPER ARTICLE No Drawing. Application February 27, 1953, Serial No. 339,472

4 Claims. (Cl. 204-11) This invention relates to an improved copper plating bath and to electroforming processes, and more particularly to an improved acid copper plating solution adapted for electroforming processes.

In many applications of copper plating it is desirable to form a firmly adherent film of copper upon a surface of a body. In other applications of copper plating it is desirable to form a layer or a coating of copper upon a surface, which coating may easily be removed from the surface. With the increasing use of electroforming techniques, this second type of application is assuming increasing importance.

Previously many methods for plating a non-adherent coating of copper upon a surface have been devised such as, for instance, providing a thin layer of graphite upon the surface to be plated. However, particularly where the final coating of copper is to be relatively thin, certain difliculties present themselves that are not readily overcome by the methods previously known.

The present invention provides an improved process and means whereby a thin film of copper may be electroplated upon a surface of a more noble metal such as platinum, palladium or gold and thereafter removed from the surface with ease. It has now been discovered that establishing and maintaining a chloride ion concentration of .025 to .045 gram per liter in a copper sulfatesulfuric acid plating bath changes the characteristics of the bath and of the metal deposited from the bath. A bath so modified not only provides a suificiently low degree of adherence between a copper film plated from the bath and the palladium or platinum surface upon which it is plated to permit easy separation, but also provides further desirable properties in the film so plated.

It is, therefore, an object of the invention to provide an improved composition for use in copper plating.

It is another object of the invention to provide an improved composition for electroforming articles of copper.

It is another object of the invention to provide an improved method of electroforming with copper.

It is another object of the invention to provide an improved copper plating bath especially suitable for electroforming thin structures.

It is still another object of the invention to provide an improved copper plating bath for electroforming upon surfaces of palladium, platinum or gold.

These and other objects will be more readily apparent and the invention more easily understood by reference to the following detailed description.

While there are many varied applications of the invention it may be sufiiciently understood by means of the single example which follows and which constitutes a preferred embodiment and application of the invention.

The practice of the invention is particularly advantageous when applied to a process of making fine mesh metallic screens such as those described in my co-pending application Serial No. 292,354, filed June 7, 1952, now Patent 2,702,270. The method as described in that application comprises scoring a grid of closely spaced Ice shallow grooves upon the surface of a glass or ceramic plate, coating the scored surface with a thin layer of wax, cathode sputtering over the wax coating a thin film of a noble metal such as palladium, removing the sputtered metal coating from the raised spaces between the grooves by rubbing the surface While it is submerged in water, electroplating upon that portion of the sputtered metal coating remaining in the grooves a thickness of metal to form a metal screen, and separating the screen from the wax-coated glass plate.

In the method described, it is important that the final metal screen formed by the electroplating process have low internal stress in order to minimize any tendency to curl or to distort. It is also desirable, when forming a screen for light-transmission purposes, that the longitudinal and cross members of the screen be as'narrow as possible, i. e., they should have more nearly a circular than an oval, rectangular or flat cross-section. It is further desirable that when separating the screen from the underlying surface the electroplated screen alone is removed. All these advantages are accomplished by means of the invention.

A preferred embodiment of the invention may be practiced according to the procedures described in my copending application referred to above using noble metals such as palladium or platinum in the sputtering operation and electroplating in a plating bath of the following composition:

CuSO4 200 grams per liter. H2804 Suflicient to adjust pH to 0.9. 01- 0.035 gram per liter. H2O Balance.

The plating is carried out in a still tank at room temperature at an average current density of 30 to 35 amperes per square foot of cathode surface area. Agitation may be used and the current density is not critical. While 30 to 35 amperes per square foot has been found to provide a preferred rate of deposition, current densities of 5 to 70 amperes per square foot also give satisfactory results when plating without agitation. The time of plating depends, of course, upon the thickness of the final film desired. A plating time of approximately 13 minutes when plating at the preferred current density provides a copper mesh screen of desirable thickness, about .0002". Following the plating, the copper screen is lifted gently from the surface, rinsed in water and dried between two sheets of paper in a current of hot air.

The plating solution may be made up in any suitable manner that provides for a relatively high degree of purity. Generally, commercial grades of crystalline copper sulfate and of sulfuric acid are suitable, and may be dissolved directly in the water. Filtration and electrolytic purification are also desirable prior to use of the bath for electroforming. Generally, the chloride ion may be added as hydrochloric acid or as any suitable acid-soluble chloride salt, such as copper chloride. Since ordinary tap water contains some chloride ions, and additional chlorides may be introduced into the plating bath by means of articles being transferred from one bath to another in the plating room, it is advisable to provide an analytic procedure to determine the chloride concentration in the plating bath both before the initial addition and periodically during long-continued operation.

The action of the chloride anion in the copper bath is not definitely understood. However, it appears not only to provide for ready separation of the plated copper from noble metals such as palladium or platinum but also aflects the shape of the mesh wires making them narrower and rounder than similar mesh wires formed in other baths. This can be seen by microscopic examination and also by the fact that a SOO-mesh screen electroformed in a bath according to the invention, when employed in a light device, will yield a light transmission value about 8% higher than a similar screen formed from any other known copper plating bath.

It should be understood that the practice of the invention is not limited to the electroforming of copper screens as described above, but is also generally applicable to any electroforming process where it is desired to produce an electroforrned body of copper containing low internal stresses. The invention also provides means for producing electrodeposits ofcopper having relatively smooth surfaces, sharp margins and minimum treeing and nodulation.

it should also be understood that the practice of the invention is not limited to the particular bath composi .tion described above, but gives advantageous results throughout the range of copper sulphate and sulfuric acid concentrations generally useful in electroplating practice. The approximate composition limits are:

CuSO4 100 to 250 grams per liter. H2504 To adjust pH to 1.5 to 0.2. Chloride ion .025 to .045 gram per liter. Water Balance.

The temperature also is quite variable, the invention being practicable over a range of operating temperatures from about 10 C. to about 50 C. Suitable agitation of the bath vor of the work in the bath may also be provided. Generally, an increase in the temperature of the bath, within the range given, and an increase of agitation, permit operation of the bath at increasingly higher plating speeds.

There has thus been described an improved acid cop per plating bath comprising from .025 to .045 gram per liter of chloride ion, and being particularly suitable for use in electroforming processes.

What is claimed is:

l. A method for electroforming an article in a copper plating bath comprising maintaining the composition of said bath between the following limits:

CuSO4 100 to 250 grams per liter H2804 Sufficient to adjust pH to 1.5 to 0.2 Chloride ion .025 to .045 gram per liter Water Balance,

immersing a body having a surface of a noble metal in said bath, passing an electric current through said bath and said surface in a direction to make said surface a cathode at a rate of 5 to 70 amperes per square foot of the area of said surface, maintaining the temperature of said bath at 10 C. to C., and removing the resulting copper deposit from the cathodic surface.

2. A method for electroforming an article in a copper plating bath comprising maintaining the composition of said bath between the following limits:

CuSO4 to 250 grams per liter H2804 Sufficient to adjust pH to 1.5 to 0.2 Chloride ion .025 to .045 gram per liter Water Balance,

said bath between the following limits:

CuSOi 100 to 250 grams per liter H2804. Sufficient to adjust pH to 1.5 to 0.2 Chloride ion .025 to .045 gram per liter Water Balance,

immersing a body having a surface of a noble metal in said bath, passing an electric current through said bath and said surface in a direction to make said surface a cathode, and removing the resulting copper deposit from the cathodic surface.

4. A method for electroforming an article in a copper plating bath comprising maintaining the composition of said bath at about:

01804. 200 grams per liter H2804 Suflicient to adjust pH to 0.9 Chloride ion .035 gram per liter Water Balance,

immersing a body having a surface of a noble metal in said bath, passing an electric current through said bath and said surface in a direction to make said surface a cathode, and removing the resulting copper deposit from the cathodic surface.

References Cited in the file of this patent UNITED STATES PATENTS 2,602,774 Beaver July 8, 1952 2,702,270 Donahue et al Feb. 15, 1955 

1. A METHOD FOR ELECTROFORMING AN ARTICLE IN A COPPER PLATING BATH COMPRISING MAINTAINING THE COMPOSITION OF SAID BATH BETWEEN THE FOLLOWING LIMITS: 